Contact-sensing probe

ABSTRACT

A contact-sensing probe has a movable member including a stylus and is supported on a fixed member at two spaced-apart locations. At each location an a spherical element on the one member is situated between convergent surfaces of the other member. A spring urges the movable member into engagement with the fixed member so that at each location the element slides on the convergent surfaces and by virtue of the convergence thereof attains a positive rest position therebetween. The movable member is displaceable from the rest position against the force of the spring when a force is applied to the stylus and is returnable into the rest position by the spring. The elements and surfaces may be electrical contacts, displacement of the stylus being sensed by any one of the contacts being broken. Alternatively displacement is sensed by a proximity sensor.

CROSS REFERENCE TO RELATED APPLICATION This is a continuation ofapplication Ser. No. 727,616 filed Sep. 28, 1976 which in turn is acontinuation in part of U.S. patent application 550,634, filed Feb. 18,1975, which in turn is a continuation of U.S. pat. application Ser. No.398,831 filed Sept. 19, 1973, all of said prior applications having beenabandoned. SUMMARY OF THE INVENTION

This invention relates to a contact-sensing probe for use in machines,e.g. inspection machines adapted to signal the co-ordinate position of astylus when the latter makes contact with a work piece and in this wayprovide a measurement of the work piece.

Such a probe necessarily comprises a movable member including a stylusand supported for movement on a fixed member, and further comprises ameans for sensing a displacement of the movable member relative to thefixed member.

It is an object of this invention to provide a contact-sensing probewherein the movable member is biased into a positive rest position fromwhich it is displaceable by a force acting on the stylus but into whichit can return with a high positional accuracy i.e. a high degree ofrepeatability of position.

It is also an object of this invention to provide a contact-sensingprobe wherein there is only a single movable member thereby to providesimplicity of construction.

It is a further object of the invention to provide a contact-sensingprobe which is free from components whose wear could introduce lostmotion or could otherwise adversely affect positive location of themovable member in the rest position.

It is yet another object of this invention to provide a contact-sensingprobe which combines simplicity of construction with the ability tosense displacement of the stylus in both senses of direction of each ofthe three dimensions of space. Other objects will become apparent incourse of the specific description of examples of the invention laterherein.

According to this invention a contact-sensing probe comprises a fixedmember, a movable member including a stylus and supportable on saidfixed member at two spaced-apart locations, at each of said locationsone of the members having mutually convergent rigid surfaces and theother one of the members having a locating element engageable betweensaid surfaces, bias means for urging the movable member into engagementwith the fixed member, the bias means co-operating with the convergentsurfaces so that at each location the locating element tends to slide onthe surfaces into a positive rest position thereon, the locatingelements and the surfaces co-operating to displace the movable memberfrom the rest position in opposition to the bias means when a force isapplied to the stylus, and means for sensing a said displacement.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of probes according to this invention will now be describedwith reference to the accompanying drawings wherein:

FIG. 1 is a sectional elevation of a probe according to a first example.

FIG. 2 is a section on the line II--II in FIG. 1.

FIG. 3 is a section on the line III--III in FIG. 2.

FIG. 4 is a sectional elevation of a probe according to a secondexample.

FIG. 5 is a section on the line V--V in FIG. 4.

FIG. 6 is a perspective view of a probe according to a third example.

FIG. 7 is an enlarged section on the line VII--VII in FIG. 6.

FIG. 8 is a section on the line VIII--VIII in FIG. 7.

FIG. 9 is a perspective view of a probe according to a fourth example.

FIG. 10 is an elevation of a probe according to a fifth example.

FIG. 11 is a section on the line XI--XI in FIG. 10.

FIG. 12 is a section on the line XII--XII in FIG. 10.

FIG. 13 is a modified detail of FIG. 10.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1, 2, 3 a probe 1 comprises a housing 2 and a stylus3. Three locating elements or arms 4 extend radially out from the styluswhich is urged by means of a spring 5 acting on the top 3a of the stylustowards a rest position defined by three locations 8 on the housing. Ateach location 8 a said arm 4 engages two mutually convergent surfaces 6,7 formed by a pair of spherical supports 9, 10 sitting in respectivedrillings 11, 12.

The end 13 of the stylus which extends through the housing terminates ina ball 14, the position of whose geometrical center is accurately known.

In operation the probe is connected to a co-ordinate measuring framework(not shown) for example a rectangular cartesian or spherical polarco-ordinate framework and is movable in space in any direction to sensethe boundaries of the object being measured. Contact between the ball 14and the object displaces the stylus away from its rest position againstthe force of the spring 5.

The displacement is detected by an electric circuit 15 which is normallycompleted when the stylus is in its rest position. Each support 9, 19 iselectrically insulated from the housing and from the other support inthe pair by means of insulation 16. The circuit path passes in seriesfrom one location 8 to the next adjacent one, and each arm, which isinsulated from the stylus and the other arms, completes the circuitbetween the two surfaces 6, 7 of each location 8. Displacement of thestylus away from its defined rest position results in at least one ofthe arms breaking contact with at least one of said surfaces of one ofsaid locations thus breaking the circuit 15 and extinguishing theindicator lamp 17, which is usually lit by the power supply 18.

The breaking of the circuit is used to provide an input pulse to anautomatic switching arrangement, not shown but well understood per se,and which may be used as an instruction to a device to read and recordthe co-ordinates of the probe at that time. It has been found that apulse of one microsecond duration enables a measurement to be made withgreat accuracy.

A clearance 19 is provided between the stylus and the housing and thisallows limited displacement of the stylus within the housing. Anydisplacement of the stylus away from the defined rest position isresiliently opposed by compression of the spring 5; thus damage to thestylus due to the inertia of carriages on which the probe is supportedmay be avoided. If inertia of the carriages is a problem, for example itmay prove difficult to prevent bending the shaft 20 of the stylus aftera reading has been taken, this can be overcome by providing a known kindof break mechanism into the sytlus.

In setting up the probe it is necessary to ensure that each arm 4 makesgood contact with each of its respective pair of supports. This canreadily be done by mounting the part 21 of each arm that fits into thestylus and coating it with adhesive. The probe is then assembled withthe adhesive still soft, and a cover plate 22 which acts as a retainerfor the spring 5 is attached by set screws 25. When the adhesive has setand the circuitry is connected to the power supply, lighting of theindicator lamp indicates that the three arms 4 have all bedded in withtheir respective supports. In this manner the manufacturing tolerancesassociated with the depth of the drillings 9, 10 in which each supportis placed are rendered harmless. To ensure that the geometric center ofthe ball 14 is in the desired place relative to the probe, provision canbe made for attaching the ball and its associated shaft 20 to the stylusafter the geometry of the members has been fixed.

The shaft 20 of the stylus is fitted by means of a spigot 24 on theshaft engaging a counterbore. The center of the ball 14 can be alignedwith the housing by making the spigot 24 a loose fit in the counterborein the stylus and adhering them together while the probe and ball 14 areheld in an assembly jig.

The drillings for the ball bearings are continued through the housing ata diameter 26 less than that of each ball bearing so that duringassembly a faulty ball bearing can be readily removed and replaced.

The shaft of the stylus as illustrated in FIG. 1 is substantiallyco-axial with the housing but it will be appreciated that this is notnecessary to the functioning of the device, for example during cylinderbore measurement it may be preferable for the ball and shaft to lieradially of the illustrated position.

It has been found that the performance of the probe is improved if it isfilled with an electrically insulating oil. This prevents sparking atthe contacts leading to deterioration thereof. The rubber gaiter 27prevents the escape of oil along the shaft and is sufficiently flexibleso as not to interfere with the seating of the probe in its defined restposition.

The electrical connections between the locations 8 may conveniently liein an annular groove 28 in the housing. It will be appreciated that theuse of spherical supports to form the convergent surfaces 6, 7 is merelyfor convenience and that each pair of such surfaces could for instancebe replaced with a pair of cylindrical supports inclined together. Itwill be clear that the contacting surfaces of the locating elements andthe support elements are mutually convex.

In this embodiment it will be understood that the rest position of thestylus is defined by, and movement from this rest position is detectedby, the same means.

In FIGS. 4 and 5 the stylus has three generally circular legs 81, 82 and83 each of which co-operates with locations formed as described withreference to FIGS. 1, 2 and 3. One of the three legs 83 is extended todefine the stylus and carries the ball 14 for contact sensing. Thisparticular embodiment may have advantages in certain circumstances forexample for the measuring of cylinder bores.

Referring to FIGS. 6, 7, 8 a probe 1 comprises a stylus 3 located withina housing 2. A hairpin spring 5 is secured at one end to a dielectricbody 91 by means of two straps 92 and 93 bonded into the dielectricbody. The dielectric body further illustrated in FIGS. 6 and 7 isgenerally semi-cylindrical with a V-shaped groove 94 extending over mostof its length and a platform portion 95 to which the straps 92 and 93are bonded. The dielectric body which is made in a polyester resin isbonded with further polyester resin into the housing 2. At two spacedapart locations respective pairs 96, 98 and 97, 99 of cylindricalsupports are bonded to the sides 112, 113 respectively of the V-shapedgroove 94 to form convergent surfaces and at a third location acylindrical support 100 is bonded to the dielectric body so as tosubtend an angle of about 45° with the axis 101 of the probe 1.

In operation the spring 5 which passes through a diametral hole 102 inthe stylus, urges two locating elements constituted by longitudinallyspaced cylindrical portions 104, 109 of the stylus into contact with theconvergent surfaces, and the curved end 103 into contact with theinclined surface of the support 100 thus defining a rest position forthe stylus. Contact between the ball 14 at the end of the stylus and anobject to be measured deflects the stylus changing its orientationrelative to the housing 2 and causing it to move away from at least oneof the supports. Thus by providing an electric circuit between thesupports and the stylus the movement of the stylus away from its restposition can be determined, each point of contact between the stylus andthe supports acting as a switch for making or breaking the electriccircuit. All the five supports 96-100 are bonded to the dielectric bodywith polyester resin and are physically separated and hence electricallyinsulated from each other. The cylindrical portion 104 is insulated fromthe remainder of the stylus by a cylindrical sleeve 105 of polyesterresin.

The electric circuit for determining when the stylus breaks contact withone of the supports is connected from the wire 106 via the support 96,the stylus 3, the support 98, the support 99, the cylindrical portion104 of the stylus, the support 97 and the wire 107 in series with apower supply and an indicating lamp (not shown).

The two wires 106 and 107 are mechanically supported relative to theprobe by a rubber cap 119 which connects to the probe by a screw thread108.

There is no need for an electrical connection to be made between thefifth support 100 and the stylus as any axial movement of the styluscauses its spherical end 103 to co-operate with the inclined needleroller 100 and thus break the electrical connection between one of theother needle rollers and the stylus.

It will be readily appreciated that although, in this embodiment, it isconvenient to use cylindrical supports to define a pair of V-shapedslots for locating the stylus in its rest position, other methods oflocation may be used. For example each cylindrical support could bereplaced by a rectangular metallic strip. Cylindrical supports arepreferred as they are readily commercially available and have a highstandard of surface finish and hardness thus yielding low frictionalforces to resist movement of the stylus and improving the sensitivity ofthe probe. The invention contemplates that any means of ensuring adefinite rest position for the stylus may be substituted for thecylindrical supports and that the present point contact between thestylus and each support may be replaced by a line contact.

In a modification (not shown) the fifth support 100 is set perpendicularto the axis 101 of the probe whereby only the two portions 104, 109 aresupported by convergent surfaces and the capability of the probe isrestricted to two dimensions.

In an alternative embodiment illustrated in FIG. 9 the same dispositionof supports 96-100 is adopted as in FIG. 6 but their function, incombination with the spring 5 is purely to locate the stylus in its restposition. An electrical circuit 15, is then completed by way of the wire112, a switch terminal 110, the stylus 3, the hairpin spring 5 and thewire 113. Any movement of the stylus away from its rest position willresult in the switch formed between the stylus 3 and the terminal 110being broken and hence said displacement can be detected via theelectrical circuit 15.

During measuring of an object with any of the aforementioned embodimentsa series of readings are taken at various positions about the object toproduce information which may readily form the input data for a computerprogrammed to compare the object with a master object. Thus probesaccording to the invention may be used for measuring such objects aspipes for gas turbine engines and cylinder blocks for internalcombustion engines.

The probes herein described have been provided with a detectorcomprising an electrical switching system but it will be appreciatedthat the electrical switching system could be replaced by fluidic orother known switches and that the detector may comprise means other thana switch. In one embodiment a detector may comprise an opticalprojection system for magnifying a mismatch between two pointers, themismatch providing an indication of movement of the stylus.

It will be further appreciated that any of the above embodiments may befilled with oil in a similar manner to that described with reference toFIGS. 1, 2 and 3.

Conveniently, the cylindrical portions 104, 109 (FIG.6) define surfacesof revolution concentric about the axis 101 and linear in the directionof the axis 101. The stylus 3 is therefore movable axially, exceptinsofar as limited by the support 100. Axial movement away from thesupport is, in this case, not detected. Nevertheless, the examples ofFIGS. 6 and 9 present economical constructions suitable for manyapplications. However, in cases where it is undesirable to useelectrical contacts, for detection of stylus displacement, or where suchdisplacement has to be detectable in each sense of the three dimensionsof space, the next-following example is appropriate.

Referring to FIGS. 10-12, a movable member 9 comprises a stylus 10connected to a rod-shaped body 11 elongate along an axis 11a normallyextending in the direction of the co-ordinate dimension Z. The stylushas at the free or sensing end thereof a disc 12 whereby to contact awork piece 13. The body 11 is supported in a housing 14 provided with aspigot 15 for fixed attachment to the head of a measuring machine (notshown) supporting the probe for three-dimensional movement relative tothe work piece.

The member 9 is supported in the housing 14 at two locations A, B spacedapart along the axis 11a. At location A the member 9 embodies aspherical element 16 engaging an axi-symmetric recess 17 constituted bymutually convergent surfaces 20 of three equi-spaced balls 18 secured tothe housing. The centers of the balls lie in a plane 19 extending in theY and Z dimensions so that the axis of symmetry 17a of the recess 17 isperpendicular to the axis 11a. Thereby the element 16 is constrained bythe recess 17 to permit pivotal movement about the axis 17a butrestrains movement in the Y-Z plane.

At location B the member 9 has a cylindrical element 21 concentric withthe axis 11a and engaging a recess 22 constituted by mutually convergentsurfaces 23 of two balls 24 whose centers lie in the plane 19 on a lineperpendicular to the axis 17a. The surfaces 23 allow movement of thecylindrical element 21 in the Z direction but resist rotation of thestylus in the Y-Z plane about the center of the ball 16.

The member 9 is biased into engagement with the balls 18, 24 by a spring25 arranged between the housing 14 and the body 11 in a positionintermediate between the locations A, B. When engaged with all fiveballs 18,24 the member is said to be in a rest position and it ispositively retained in that position by the bias created by the force ofthe spring and the convergence of the ball surfaces which bias urges theelements 16, 21 down into the lowest possible position in the recesses17, 22. As will be explained, the member 9 is displaceable from the restposition in opposition to the force of the spring by a force applied tothe disc 12 by engagement thereof with the work piece and the member 9will be returned to the rest position by said bias on cessation of thelatter force.

Any movement of the member 9 relative to the housing 14 is detectable bya metal proximity sensor 26 provided in a resin part of the housing. Forthe purpose of detecting such movement use is made of the fact thatmovement of the disc 12 in any of the dimensions X, Y, Z must result intilting of the member 9 such that axis 11a thereof moves out ofparallelism with the Y-Z plane. Thus, movement in the Y direction causesthe cylindrical part 21 to slide up one or other of the surfaces 23 andso pivot member 9 about the axis 17a and out of the Y-Z plane. Similarlymovement in the Z direction causes the ball 16 to slide up at least oneof the three balls 18 and tilt the member 9 on the balls 24. Movement inthe X direction causes the member 9 to rock on the balls 18 or the balls24. Any of these movements is effectively detectable at a portion 27 ofthe body 11 substantially medially between the locations A, B and thesensor 26 is mounted accordingly in a position adjacent the portion 27and such as to detect movement thereof perpendicular to the Y-Z plane.

It will be seen that movement in any of the directions X, Y, Z may takeplace in either sense of direction. For example, movement of the disc 12in the Z direction may take place towards or away from the housing. Thelatter situation applies when the machine is used to raise the disc 12against an undercut surface 13a of the work piece.

When, after contact with the work piece, the probe is moved so thatcontact with the work piece is disestablished and the force on the disc12 ceases, the bias created by the spring and the convergent surfacescauses the elements 16, 21 to slide back into the lowest position in therecesses 17, 22 so that the member 9 is returned to the rest position.

To ensure good sliding action and high repeatability in attaining therest position the elements 16, 22 and the balls 18, 24 are made ofhardened and polished steel. Commercial ball bearing finishes areusually sufficient for this purpose.

The sensor 26 is known per se as comprising for example a pair ofelectro-magnetic induction coils 28, 29 connected in a bridge circuit 30such that a change in the reactance of one of the coils, as may beproduced by a change of position of the stylus, varies the bridgeoutput, e.g., away from zero. The bridge output is taken through anamplifier 31 to a suitable indicating means, e.g. a volt meter.

The measuring machine referred to is known per se and generallycomprises a head supported by carriages movable respectively in the X, Yand Z direction respectively. The carriages are provided with means formeasuring the co-ordinate position of the head and thus of the probeattached thereto. A work piece is measured by measuring the position ofthe head at the instant when, on contact between the disc 12 and thework piece 13, the output of the sensor 25 changes.

It will be clear that in the absence of provision to the contrary themember 9 is rotatable about the axis 11a. This is no disadvantagebecause if the sensing end of the stylus is rotationally symmetricalabout the axis lla, e.g. as in the case of the disc 12, the angularposition of the member 9 about the latter axis is immaterial. However ifthe sensing end of the stylus is laterally off-set from the axis lla, asshown by a lateral arm 32 and spherical end 33, (FIG. 13) the needarises for inhibiting rotation of the member 9 about the axis 11a.

As shown in FIGS. 10 and 12 the member 9 has secured thereto a lateralarm 34 the free end 35 of which is provided with a convex surfaceresting at a location C, on a flat surface 36 on the housing 14. Thespring 25 is connected between the arm 34 and the housing in a positionrelatively closer to the end 35 than the axis 11a because the biaspressure required at the locations A, B is smaller than at the locationC. However, separate springs may be provided at the location C anddirectly intermediate between the locations A, B.

The term "axisymmetric recess" used herein is intended to include aconical recess which may be used instead of the recess formed by theballs 18. At each location A or B the recess 17 or 23 may be provided oneither one of the members 9, 14, the element being provided on thecorresponding other one of the members 9, 14. Instead of the one sensor26 shown, two such sensors may be provided respectively adjacent thelocations A, B.

Referring further to FIGS. 10 and 11, the convergent surfaces 20, 23 ofthe fixed member 14 may be termed "first surfaces" and the regions ofthe movable member 9 engageable with the surfaces 20, 23 in the restposition of the movable member may be termed "second surfaces" which areslidable on the first surfaces.

To ensure point contact, or at least line contact, between a first andsecond surface they must be mutually convex. This condition is satisfiedif both surfaces are convex or one is flat while the other is convex.

We claim:
 1. A device for mounting a stylus in a position-determiningapparatus wherein said device and an object are movable relative to eachother for providing a signal when said stylus engages said object,thereby indicating the position thereof, said device comprisinga fixedmember, a movable member supportable on said fixed member at twolocations spaced apart along a first axis, the movable and fixed membersrespectively defining at each said location a supported elementconfronting a supporting element in the direction of a second axistransverse to said first axis, at each said location one of thesupported and supporting elements having surface means convergent in adirection of said second axis and the other one of said elements beingengageable with said surfaces, bias means for urging the movable memberinto contact with said fixed member, all of said convergent surfacemeans thereby positively defining a rest position for said movablemember and said movable member being removed from said rest position inopposition to said bias means when a force is applied to said stylus andsaid bias means and convergent surface means cooperating on cessation ofsaid force, to return said movable member to said rest position, asingle proximity sensor having a fixed and a movable element having agap therebetween and having an output corresponding to the magnitude ofsaid gap, said fixed and movable element being situated on the fixed andmovable member respectively in positions intermediate between saidlocations, said supporting elements of the fixed member and said fixedelement of the sensor being situated at the same side of said movablemember, whereby any said movement of the movable member away from saidrest position results in increasing said gap and correspondinglyproduces a change in said sensor output.
 2. A device according to claim1, wherein the convergent surface means at the one said location arepositioned to bias the respective supported element against movement ina plane perpendicular to said second axis, the convergent surface meansat the other said location are positioned to bias the respective supportelement only against movement in the direction of a third axisperpendicular to said first and second axes, whereby said movable memberis removable from said rest position by a force applied to the stylus ineither sense of direction along said first axis, and said bias means andthe convergent surface means at said one location cooperate to returnthe movable member to the rest position when said force ceases.
 3. Adevice according to claim 2, further comprising means for opposingrotation of said movable member about said first axis, and said stylushaving a portion offset from said first axis for the application to thestylus of a force directed along said first axis but in the sense awayfrom said locations.
 4. A device for mounting a stylus in aposition-determining apparatus wherein said device and an object aremovable relative to each other for an object-contacting portion of saidstylus to engage the object, the device having an axis and theobject-contacting portion of the stylus being offset from said axis, thedevice comprising a fixed member, a movable member to which said stylusis connected, first surface means on one of said members, engageable bythe other one of said members and positioned to constrain said movablemember against movement in one sense of direction along said axis,second surface means on one of said members, engageable by the other oneof said members and positioned to constrain said movable member againstmovement along said axis in the sense of direction opposite to said onesense, biasing means for urging said movable member into engagement withsaid fixed member at said first and second surface means respectively,said first and second surface means cooperating to define a positiverest position for said movable member on said fixed member, said movablemember being displaced from said rest position in opposition to saidbias means when a force is applied to said object-contacting portion ofthe stylus, and said biasing means cooperating to return said movablemember to said rest position when said force ceases.
 5. A deviceaccording to claim 4, further comprising means supporting the stylus formovement transverse to said axis, and means biasing the stylus againstsaid transverse movement and into a rest position.
 6. A device accordingto claim 4, wherein said first and second surface means comprise anaxisymmetric recess provided at one of said members, and the other oneof said members comprises a spherical element engagable with saidrecess.
 7. A device according to claim 6, further comprising means forsupporting the movable member at a location spaced from said recess inthe direction of said axis.